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Collapses equal to 2.0 quake

By Greg Botelho

(CNN) -- The attacks on the World Trade Center literally shook New York -- the equivalent of a 2.0 magnitude earthquake -- but should not weaken structures in lower Manhattan, experts said.

"This is the most significant collapse of a building in history," said Art Lerner-Lam, Associate Director for geology and geophysics at Colombia University's Lamont-Doherty Observatory. "But the (seismic) impact is relatively mild."

While the Richter Scale specifically applies to earthquakes, Lerner-Lam said Lamont-Doherty seismologists calculated that both planes' impacts into the two World Trade Center towers would have registered a magnitude of 0.8.

The eventual collapses, he said, would have measured a magnitude of 2.0.

No information was available what the seismic repurcussions would be if two nearby building affected by flying debris and resulting fires -- a 54-story structure called One Liberty Plaza or the trade center complex's 9-story building 5 -- collapsed

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Such high measures are extremely abnormal, if not unprecedented, Lerner-Lam added. The 1993 World Trade Center bombing and Tuesday's collapse of building 7 at the complex barely registered at all, he noted.

The seismic activity, as well as video showing the towers' collapse, led experts to believe fire -- not the planes' impact with the structures -- felled the buildings.

"If just the airplane impact had occurred, and there wasn't any fire, I don't think the buildings would have collapsed," said John Filson of the United States Geological Survey. "The fire here was the terrible thing that weakened the structure, weakened the floor where this was hit."

Aaron Swirsky said "nobody could foresee something like" what unfolded Tuesday when he and 13 other architects designed the World Trade Center in the early 1970s.

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